1. Field of the Invention
The present invention relates to the field of sailboats having more than a single hull, such as catamarans and trimarans. More particularly, the invention relates to a design modification that enhances the practicable speed that can be obtained from a small multi-hull sailboat. More particularly yet, the invention relates to such a modification that enhances the ability of the boat to plane.
2. Prior Art
Many serious sailors seek very high performance in a sailing dinghy. Ideally, the dinghy is relatively inexpensive and is easily loaded on top of a vehicle and transportable overland. Unfortunately, the highest-performance sailing dinghies, such as the WindRide RAVE, are both expensive and difficult to transport. Although the most popular relatively-low-cost sailing dinghy, the LAZER, is fast, it is not extremely fast and, furthermore, it is cumbersome to transport.
There are two main design approaches to increasing the speed at which a sailboat can go: increase the force of the wind against its sails; decrease the drag to which its hull is subject. At low speeds, hull drag is dominated by frictional forces across the entire hull/water interface. Efforts to minimize it include designing and maintaining a smooth hull surface. At speeds higher than about 1.34×L1/2, where L is the length of the hull at the water line (often designated D.W.L.), hull drag is dominated by the hull's wave-making. Hull-created waves are of two types: (a) long wavelength disturbances created by the hull's lateral displacement of water it moves through it, waves that propagate perpendicular to the boat's direction of movement; (b) short wavelength turbulence that appears immediately behind the stern. If and when the boat begins to rise up in the water to a planing configuration, reducing the volume of water the hull must displace as it moves forward, drag due to wave generation is greatly reduced. When the hull is planing it is in effect skimming along the water's surface rather than plowing through the water. The more readily a boat will plane, the higher the speed it can attain with given sail area and wind.
In effect, a boat planes—moves up on top of the water—because it is moving so fast that the water cannot get out of its way, just as an automobile tire rides up onto a thin film of water if it is moving fast enough, a phenomenon referred to as hydroplaning. In each case, the water under the object exerts an upward force on the object at the object/water interface that is equal to the weight of the object. In the case of the tire, this is about ¼ the weight of the car. It can also be stated in terms of equating the pressure exerted upward by the water to the pressure exerted downward over the total object/water contact area. The upward pressure that the water can exert against an object moving across its surface is proportional to the square of the speed of the object. Thus, in the case of the automobile tire, where the contact area remains essentially the same as the tire begins to hydroplane, the speed at which hydroplaning commences is proportional to the square root of the tire's air pressure. VPlane=Const.p1/2. In the case of a boat's hull, the contact area tends to become smaller as the hull rides higher in the water. Also, since the boat's weight tends to increase with length, the general expression for planing speed of a boat is stated in terms of its length, even though ultimately it is the pressure that is crucial. A typical general expression for the threshold speed for a boat's planing is VPLANE=2.5L1/2, where L, the waterline hull length is given in feet, and the speed will be expressed in knots. The exact speed at which planing will commence will vary from boat to boat, depending on the shape of the hull. If one wishes to enhance the planing ability of a boat of given weight, that is, to lower the speed at which it starts to plane, one flattens the hull. If all other things are equal, this will minimize the pressure for a given weight.
Of course, other things are not all equal; and shifting a sailboat's hull shape away from the traditional contoured configuration and toward a flat-bottomed shape greatly increases the boat's vulnerability to capsizing as the wind speed increases, a serious impediment to attaining high speed. Thus, if one wishes to enhance the planing capacity by going to a flat-bottomed hull, other changes must also be made to the boat's design.
A boat moving slower than its planing speed travels through the water in “displacement mode.” Large mono-hull sailboats tend to always operate in displacement mode, being unable to reach planing speeds because of the shape of the hull and the tremendous residual wave-making forces, which increase exponentially with increasing speed.
For a given hull design, it would seem that deploying a greater sail area would increase the ability of the boat to plane. However, this results in a greater overturning moment, tending to capsize the boat, both because of the greater force and because of the generally greater moment arm as the masts are increased in height in order to accommodate more sail. So, again, a straightforward approach to improving the planing capacity results in an increased likelihood of capsizing the boat, or at least of a wind-dumping blow-down, before the planing speed is attained.
In a standard single-hull sailboat, the wind-generated overturning moment causes the hull to heel toward the leeward side. If the boat has a heavy keel, this heeling forces the keel upward and windward, automatically producing a righting moment that counters the overturning moment. Ideally, the righting moment and the overturning moment are in dynamic equilibrium and the boat moves along without capsizing. The hull shape may also contribute to the righting moment, thereby allowing the boat to take advantage of greater wind forces. This is accomplished by designing the hull so that as it heels, it displaces more water, thereby increasing buoyancy on the leeward side and adding to the righting moment. Furthermore, the crew of the vessel also provide an adaptable righting moment by being able to lean out, or hike out, from the deck on the windward side, thereby adding their weight to the righting moment. However, this in general is a significant righting-moment contribution only for small boats with small or absent keels.
In contrast to mono-hull boats with keels, catamarans gain their righting moment primarily by the spread between the two hulls, so that an entire hull is always a large distance to leeward from the center of mass, thus giving a large moment arm (with respect to the boat as a whole) on which the buoyant force on that hull operates. The hulls of a conventional catamaran are narrow, and shaped to minimize wave-making; they are sometimes referred to as wave-piercing hulls, because of their ability to cut through waves. As such, they are fast hulls, although they are incapable of planing because of this very sharpness. Because of the strong righting moments, catamarans are able to support a large sail area relative to their weight. Large catamarans are therefore generally faster than mono-hull sailboats of similar weight.
Nevertheless, the speed advantages of the catamaran in comparison with mono-hull sailboats largely disappear when a small catamaran is compared with a small mono-hull boat. This is because a small mono-hull sailboat, with a shorter, lighter hull, is able to plane at wind speeds that are typically available. Furthermore, a small boat does not need to carry heavy fixed ballast, because of the ability of the crew to serve well as movable ballast. Given that the crew hike out to windward as needed, the weight of a typical crew can generate the righting moment produced by a keel of much greater weight and thus be more than adequate to counter the overturning moment generated by the wind on the sail area deployed by a small mono-hull boat. The result is that a small mono-hull sailing dinghy can deploy a great area of sail without requiring a heavy keel. Without the need for a heavy keel, the small sailing dinghy can have a light flat hull capable of planing at low speed and of exerting very low drag while planing.
Attempting to negate the advantages of the small mono-hull sailboat by simply equipping a catamaran with wide flat hulls will not work, since the slightest heeling causes a corner of the flat hull on the leeward side to dig into the water, destroying the planarity of the hull contact with water. For these various reasons, small high-performance mono-hull boats are often faster than small catamarans.
If it were possible to maintain the catamaran hull flat on the water while the mast and sail react to the wind's forces, greater use could be made of those forces in driving the vessel to higher speeds. Traditionally, the hulls, the platform, and the mast move together as a unit. In the absence of wind, the platform is parallel to the water, and the mast is vertical. When the wind blows, the mast tips from the vertical to the leeward and the platform tips from the horizontal, so that the windward side goes up and the leeward side down. The angle at which the hulls intersect the water changes commensurately. Of course, as noted above, the hulls on catamarans are rounded; therefore, even when the boat is upright, no hull surface is parallel to the water surface.
There have been attempts in the past to de-couple the orientation of the mast from that of the hulls and platform. The goal of these attempts, however, was simply to improve the stability of the craft for the comfort and safety of its occupants, rather than to permit increased speed. Sundelin (U.S. Pat. No. 3,696,772; issued Oct. 10, 1972), although not directed at multi-hull vessels, per se, addresses some of the stability problems of such vessels, teaching a variably positioned outrigger device that effectively converts a mono-hull boat, including canoes and other non-sail-powered craft, into a three-hull sailboat. The gist of Sundelin is that the outriggers can be shifted with respect to the longitudinal axis of the central hull so that a longer righting-moment-arm can be deployed leeward. In particular, by changing the angle at which the outrigger-coupling rod encounters the mast, that part of the boat holding passengers can be maintained parallel to the water's surface even as the mast heels over as if in a blow-down. In other words, the device of Sundelin is directed entirely to the safety and comfort of passengers, and has no features that enhance the speed of the craft. Rather than being configured to take maximum advantage of high winds, it is designed to dump the wind without the hull heeling over significantly. Sundelin views extreme heeling over as a safety and comfort problem. In the present context, extreme heeling over, so that the craft capsizes or dumps wind, is viewed as a speed problem, one that robs the boat of the speed that it might otherwise attain.
Riordan (U.S. Pat. No. 4,192,247; 1980) discloses a trimaran that has braces extending from the mast on the center hull to the trimaran's outer hulls, which are conventionally narrow and rounded. The bracing system increases the available righting moment of the vessel. It does nothing, however, to enable the vessel to plane as a means of attaining higher speeds.
Therefore, what is needed is a multi-hull vessel that takes full advantage of available wind, regardless of its size. What is yet further needed is such a vessel that is easily transportable.